1. Field of the Invention
The present invention relates to an X-ray intensifying screen and a phosphor for an X-ray intensifying screen used in medical radiography or industrial radiography.
2. Description of the Related Art
An X-ray intensifying screen generally improves sensitivity of a photosensitive material for use in medical radiography when it is used in combination with an X-ray film. This X-ray intensifying screen generally comprises an intensifying screen base, a phosphor layer coated on the intensifying screen base and containing an organic binder and a phosphor, and a transparent protective film having a thickness of about 10 .mu.m for covering the phosphor layer. An X-ray film is sandwiched between a pair of two sheets of the X-ray intensifying screen so that a photosensitive emulsion layer formed on each side of the X-ray film and having a thickness of about 20 to 30 .mu.m is brought into contact with the protective film of the X-ray intensifying screen. In order to improve sharpness of an X-ray image, it is important that the X-ray film and the intensifying screen are in tight contact with each other.
A phosphor for use in the X-ray intensifying screen of this type is required to have a high luminous efficiency, a large X-ray absorption amount, and an emission spectrum suitable for spectral sensitivity of an X-ray film.
In general, an X-ray intensifying screen coated with a phosphor having a large X-ray absorption amount has good graininess of an X-ray image and improves diagnostic performance in medical radiography. In addition, when a phosphor having a high luminous efficiency is used, sensitivity of a photosensitive material is increased. Since, therefore, photography can be performed with even a small X-ray radiation amount, an exposure amount of an object to be examined can be reduced.
If an emission spectrum is unsuitable for spectral sensitivity of an X-ray film, a part of light emitted from an X-ray phosphor is not absorbed by a photosensitive emulsion layer coated on the input side of the film but scattered and transmitted. This scattered and transmitted light produces light to be absorbed by an emulsion layer on the opposite side, i.e., so-called crossover light, thereby reducing sharpness of X-rays.
In order to obtain a high-quality image, therefore, it is essential to improve three factors of sharpness, graininess, and sensitivity. For example, however, when sensitivity is increased, sharpness and graininess are decreased. Therefore, it is very difficult to maintain all of the three factors at high level.